This application claims the priority of German Patent Application No. 198 34 542.9, filed Jul. 31, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for manufacturing bodies containing reinforcing fibers. Fibers, fiber bundles, or fiber agglomerates containing at least one binder that can be carbonized, and possibly additional fillers or additives, are mixed into a molding material. The molding material is then compressed to form a body containing reinforcing fibers using a mold having a cavity closely resembling the final shape.
The method according to the present invention is used in particular for manufacturing carbon-fiber-reinforced (CFR) ceramic composites. For this purpose, fabric or mats made of carbon fibers or fiber bundles, possibly coated with binder, are mixed together with other binders and fillers. The mixture is molded to form a so-called green compact or CFR composite and is then pyrolyzed. The binder or at least one of the binders changes to a porous carbon matrix. The resultant porous carbon or carbon-carbon body is then melt-infiltrated with silicon. The present invention relates to the manufacture of the green compact or CFR composite.
DE 44 38 455 C1 is directed to the manufacture of vented brake disks in which the vent channels are created by joining fiber-reinforced bodies of different geometries. The problem with this method is that fiber fabric or fiber mats are used that are saturated with binder and placed on top of one another. This produces an anisotropic body whose fibers are oriented in two preferential directions corresponding to the fabric layers. This creates problems with joining a plurality of such bodies to form a complex structure, since the connection between the individual elements is not sufficiently stable.
Instead of fiber fabric or fiber mats, unpublished German Patent Application 197 11 829 1 is directed to the use of individual fibers or fiber bundles randomly oriented in all three directions. This in turn produces an anisotropic body with properties that are essentially the same in all three directions in space. The problem remains of joining such bodies so that a stable connection is produced at the joints.
The object of the present invention therefore consists in creating a method to join two bodies with stable connections at the joints.
This object is achieved according to the present invention by compressing the molding material in a mold that has one or more inserts so that the resultant body containing reinforcing fibers has recesses matching these inserts. The reinforcing fibers are oriented along these recesses roughly parallel to the lateral edges of the recesses.
CFR composites, for example green compacts with optimized fiber orientation, are produced by the method according to the present invention. The method is characterized by the fact that the green compacts have recesses and/or projections or nubs corresponding thereto. As a result of its flowability, the molding material in the vicinity of the recesses follows the fiber alignment of the pressing force. When such bodies are joined, a solid connection at the joint results due to the fiber orientation.
The inserts in the mold comprise both (1) projections permanently connected with the interior of the mold or other uneven locations on the interior, and (2) parts placed loosely in the mold, such as bolts or inserts of any shape, for example, parallelipipeds, stars, pins, and the like.
Fibers, fiber bundles, or fiber agglomerates based on carbon, nitrogen, boron or silicon are suitable as reinforcing fibers, for example. Carbon fibers or silicon carbide fibers are particularly well suited. The fibers preferably have a length of 1 to 30 mm. The at least one curable binder that can be carbonized is preferably used in an amount of approximately 5 to 16 weight percent based on the molding material.
By varying the amount of binder and the fiber length, the flowability of the resultant molding material can be adjusted. If necessary, one or more extrusion molding aids can be used as well.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.